1. Field of the Invention
The present invention relates to a transmission line, a resonator, a filter, a duplexer, and a communication apparatus used for radio communication and for transmitting and receiving electromagnetic waves in, for example, microwave bands and millimeter-wave bands.
2. Description of the Related Art
Ordinary RF circuits employ planar circuits that use transmission lines, such as microstrip lines, which can be easily produced and that are suitable for being miniaturized and made thin.
In the microstrip line, however, current concentration due to surface effects occurs on a conductor surface. Particularly, it is apparent at the edges, causing a power loss in a narrow region in a range of several micrometers (μm) to several tens of micrometers (μm) around the edges, accounting for 50% of the entire power loss. This phenomenon, called an edge effect, is attributed to the cross-sectional shape of the conductor (electrode). In planar circuits in which electrodes, such as microstrip lines, are formed on a substrate, edges always exist. Therefore, the problem of power loss due to the edge effect always occurs and is known to be unavoidable.
In this connection, RF transmission lines for aiming to reduce the current concentration at the conductor edges were suggested as disclosed in (1) Japanese Unexamined Patent Application Publication No. 8-321706 and (2) Japanese Unexamined Patent Application Publication No. 10-13112.
In each of the above publications, a plurality of linear conductors is formed at a constant pitch, parallel to a signal-propagation direction. It can be said that, in the above-described conventional transmission lines, the conductor is divided parallel to the signal-propagation direction to reduce the current concentration at the edges. However, to form the conductors with the correct line width in these structures requires very severe manufacturing accuracy, on the same order of magnitude as the skin depth. In addition, the conductor Q value is improved only within a small range of 10 to 20% of the conventional Q value. Further, depending on the dividing method, there are cases where the conductor Q value decreases, so that it is actually lower than the Q value of a single-line conductor.
Thus, in the structure in which the direction of a current path is the same as the signal-propagation direction, even when the linewidth is divided to be as thin as possible, the left and right edges still exist. Therefore, the structures are not effective enough as a fundamental solution to the edge effect problem.